The downsizing of electronic devices brought about by the rapid development of modern industry was accompanied by a desire for additional increases in the capacity of secondary batteries. This resulted in the development of lithium secondary batteries, which have higher energy densities than nickel-cadmium batteries and nickel-hydride batteries, and efforts to improve the properties of lithium secondary batteries have also continued up to the present time.
On the other hand, against the backdrop of global concerns such as environmental issues and energy issues, substantial expectations are also coalescing around the application of lithium secondary batteries as large power sources, e.g., vehicle power sources and stationary power sources. However, it is generally essential with such batteries to ensure the stability versus repetitive charge/discharge over long-term periods of time, and in addition their use in environments exposed to the atmosphere is also on the horizon. As a consequence, the battery properties in low-temperature environments, such as below the freezing point, and particularly the low-temperature discharge characteristics are important considerations in the development of these batteries.
The components constituting a lithium secondary battery may be broadly classified into mainly a positive electrode, a negative electrode, a separator, and an electrolyte solution. Among these, the electrolyte solution commonly takes the form of a non-aqueous electrolyte solution prepared by dissolving an electrolyte, e.g., LiPF6, LiBF4, LiClO4, LiCF3SO3, LiAsF6, LiN(CF3SO2)2, LiCF3(CF2)3SO3, and so forth, in a non-aqueous solvent, e.g., a cyclic carbonate such as ethylene carbonate or propylene carbonate; a chain carbonate such as dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate; a cyclic ester such as γ-butyrolactone or γ-valerolactone; or a chain ester such as methyl acetate or methyl propionate.
In one tactic for improving the extended durability of lithium secondary batteries, a prescribed compound is added to the aforementioned electrolyte solution in order to bring about the formation on the negative electrode of a passivation film during the initial operation of the battery. This results in an inhibition of secondary reactions, such as the reductive degradation reactions of the solvent that are a major cause of deterioration.
One such compound is a compound having the isocyanate group in the molecule. Patent Document 1, Patent Document 2, and Patent Document 3 disclose improvements in the cycle stability achieved through the addition to the electrolyte solution of, respectively, an isocyanate group-containing low molecular weight compound, a chain isocyanate compound, and a diisocyanate compound.
In addition, with the goal of improving the cycle characteristics, Patent Document 4 proposes the addition to the electrolyte solution of a prescribed sulfone compound in combination with an isocyanate compound. However, it is difficult with the aforementioned isocyanate compounds in particular to satisfy the extended durability performance required in large-scale batteries, e.g., for automotive service. In addition, there is demand for additional improvements in order to achieve satisfactory battery characteristics also including the low-temperature discharge characteristics.
On the other hand, lithium secondary batteries in some cases use an immobilized electrolyte solution as provided, for example, by impregnating a non-aqueous electrolyte solution within a higher order structure formed by a polymeric matrix. This makes it possible to increase the design freedom with regard to battery shape and to provide a lithium secondary battery that is almost entirely free of fluid leakage. These so-called gel polymer electrolytes may also use an isocyanate compound. For example, Patent Documents 5, 6, and 7 disclose the formation of polymeric matrices through the curing of a combination with a polymer that can bond with the isocyanate group, e.g., a polyol.
Low molecular weight aliphatic diisocyanates and alicyclic diisocyanates as described above, as well as the polyisocyanates produced using such diisocyanates as their primary starting materials, are often used as crosslinking agents for forming polymeric matrices. Resins cured using a polyisocyanate generally exhibit excellent mechanical properties, for example, flexibility, and an excellent resistance to chemicals, and are widely used in, for example, coatings, adhesives, sealants, waterproofing agents, foams, and elastomers. They are also advantageously used in polymer gel electrolytes for the same reasons.
Polyisocyanates having, for example, a carbodiimide, uretdione, oxadiazinetrione, biuret, urethane, allophanate, or isocyanurate skeleton are known. Disclosures relative to biuret-type polyisocyanates are made in, for example, Patent Document 8 and Patent Document 9; disclosures relative to isocyanurate-type polyisocyanates are made in, for example, Patent Document 10 and Patent Document 11; and a disclosure relative to allophanate-type polyisocyanates is made in, for example, Patent Document 12.
However, when an electrolyte solution is immobilized, the polymeric matrix impairs ion mobility and the battery resistance is then substantially increased, and as a consequence this is unsuitable, for example, for batteries where large currents are required. Accordingly, and depending on the battery duty, it may be undesirable for an isocyanate compound and a compound curable therewith to both be present in the electrolyte solution and for the electrolyte solution to be immobilized by their cure.
Patent Document 1: Japanese Patent Application Laid-open No. 2005-259641
Patent Document 2: Japanese Patent Application Laid-open No. 2006-164759
Patent Document 3: Japanese Patent Application Laid-open No. 2007-242411
Patent Document 4: Japanese Patent Application Laid-open No. 2010-225522
Patent Document 5: Japanese Patent Application Laid-open No. 2005-158703
Patent Document 6: Japanese Patent Application Laid-open No. 2005-294020
Patent Document 7: Japanese Patent Application Laid-open No. 2004-214041
Patent Document 8: Japanese Patent Application Laid-open No. S63-174961
Patent Document 9: Japanese Patent Application Laid-open No. H8-225511
Patent Document 10: Japanese Patent Application Laid-open No. S63-57577
Patent Document 11: Japanese Patent Application Laid-open No. S57-47319
Patent Document 12: Japanese Patent Application Laid-open No. H7-304724